Because its an incredibly inaccurate way of potentially diagnosing an issue. The natural variation in people's body temperatures would leave a considerable number of people who are perfectly fine with having to deal with potentially being diagnosed with it, whilst at the same time a number of people with what appear to be normal average temperatures undiagnosed as they are high for the individual.
I could easily be one of the many in the latter category. My average body temp is about 2 degrees below average, so i would show up as 'normal' when i infact have a fever. In this kind of case averages are pretty useless when there is significant variance.